Paremuka Detention Ponds

Introduction

A key to Waitakere City's Green Network is the creation of a continuous corridor of green from the west coast, across the Waitakere Ranges to the urban area and the Waitemata harbour via bush, open space and streams. Another aim is to enhance access to our natural environments.

As well as restoring the Paremuka Stream and replanting its banks with native bush, this project is putting in place a number of ecologically friendly systems to control stormwater runoff and provide wildlife habitats.

A system of four ponds was designed to collect stormwater runoff from the upper Paremuka Stream catchment, in Ranui. Construction of the ponds took place over a period of two years, from 1997 to 1998. It included the construction of a large intake structure, which limits the release of water downstream during storms to a maximum rate of 33 cubic metres of water a second.

This system of detention ponds is designed to detain, purify, store and cleanse stormwater before it enters the area’s waterways.

The ponds are also a place for residents to play, relax and enjoy the bush, with a walkway along the Paremuka Stream which will provide a link up the valley to Xena Lake.

Swale drainage

Elsewhere in the Paremuka catchment, a grass swale is used instead of the more common concrete kerb and channel to control road stormwater drainage. In this system - stormwater from the road runs across the grass swale which filters sediment and pollutants and cleans the stormwater before it flows down the bank and enters the Paremuka Stream.

Restoring the shade

The clearance of lowland bush has created a water system virtually unknown in New Zealand - the sunny stream. Our native plants, used to shady, forested waterways, cannot compete in such open conditions, leaving this human-created habitat ripe for invasion by exotic weeds.

Over time the tangled stems of waterweeds reduce water speed, blanketing rocky-bottomed forest streams with silt. Poor water quality and flooding are the usual result.

Restoring the shade to Paremuka banks is essential not only for its ecological qualities but also for managing the area’s water flow.

This is achieved through careful replanting.

The secret world of a stream

The Paremuka Stream is home to many native animals and insects, despite the years of degradation  Most of these creatures are well camouflaged and signs of life are not always easy to see.

Banded Kokopu

These little fish are of the three main species commonly called whitebait and are found in large numbers in this Stream.

All whitebait, the young of native galaxiid fish, spend part of their life cycle in fresh water and part in the sea. They grow to adults in streams, lakes and swamps, feeding on small aquatic animals until autumn when a mysterious urge calls them downstream to spawn. After hatching the young fish larvae drift out to sea for roughly six months before returning to fresh water, swarming up estuaries on the rising tide.

Eels

Freshwater eels also spend part of their life cycle at sea, but instead of shedding their eggs at the river mouth adults migrate huge distances to an unknown part of the Pacific to spawn. Leaf-like eel larvae drift back with the ocean currents, changing into slender, transparent glass eels once they reach the continental shelf. These swim upstream, feeding and developing into the familiar greyish brown elvers. The eels’ diet progresses from insect larvae and snails to crayfish, trout and even duckling. You can find both short and long-finned eels in the Paremuka Stream.

Crustacea

The best time to spot freshwater crayfish or koura is at night when these bottom dwellers emerge from their hiding places under rocks or silt to scavenge for food. More frequently seen are freshwater shrimps that dart around in the water.

Exotic pests*

A major threat to our waterways is the increasing number of exotic pest fish, such as Koi carp and gambusia (mosquito fly). Koi like overgrown goldfish, can grow to 10kg. They are bottom feeders that uproot vegetation and cloud water to the point where native fish cannot grow. Gambusia, small, silvery and almost transparent, are very aggressive fish that attack native species, biting off tails and dorsal fins. Both are ‘unwanted organisms’ under the Biosecurity Act 1993.

*Please don’t empty unwanted aquariums into streams, ponds or drains.

The birds

One of the main concerns of any city-dwelling animal is finding enough food. Much of the replanted bush here is designed to provide a year-round supply of food, but residents in the area can help too. One way to encourage native birds and insects into your garden (and the reserve) is to plant some of their favourite trees and shrubs, such as karaka, rewarewa, kowhai, karamu, cabbage trees, pigeonwood, manuka and kanuka.

Another way to attract birds is to put out artificial feeders filled with fruit, nectar and seeds, especially in winter. There are two tricks to a successful artificial feeder: place it in an open space; and put food out regularly.

Birds of Paremuka

Some of the most common birds seen around the Paremuka Stream include: (native) grey warbler, pukeko, North Island fantail, kingfisher, paradise shelduck; (self-introduced) blackbird, chaffinch, myna, eastern Rosella, hedgesparrow, thrush, mallard, spur winged plover, starling, ring-necked pheasant, skylark.

Astelia Grandis Reserve

Astelia grandis or swamp astelia is found in two spots along the Paremuka Stream. Although quite hards, this flax-like, swamp-loving plant is now under threat because of habitat decline. It is currently know to grow in only three other places in Waitakere.

The plant is easily recognised by its stiffly arching leaves with a strong white vein on each side of the midrib. Flowering between October and November, Astelia grandis products produces orange berries between February and May.

It is hoped that this regionally rare plant will flourish as the Stream is restored and the newly planted native bush along its banks thickens.

New Zealand as 13 native Astelia, ranging in habitat from high county bogs to lowland swamps and tree branches (look for clumps of green, flaxy leaves in old puriri and kauri). They also vary greatly in size. Astelia linearis is the smallest with leaves 10cm long; clumps of A. grandis, the largest of the native astelia, can grow up to 2 metres high.

Tree Weta - Putangatanga

Wetas have been a part of the landscape since the dinosaurs, dwelling in prehistoric kauri forests well before the supercontinent of Gondwanaland broke apart. They belong to the same family as grasshoppers, locusts and crickets. Tree wetas do not sting, but may bite if mishandled. They are often found in kanuka and forage largely on fresh leaves, particularly karamu and mahoe. Adults grow to 50mm and although their natural habitat is forests, they have adapted remarkably well to city living. Adults seldom move more than a few metres from their home, except when females climb down to lay their 200-300 eggs in the soil.